The present invention relates generally to television signal transmission systems and particularly concerns a system for multiplexed transmission of a compressed television signal and one or more auxiliary data signals.
A number of systems have recently been proposed for digitally transmitting compressed television signals. High definition television (HDTV) and compressed NTSC, for example, employ this technology. FIG. 1 illustrates a typical prior art arrangement of such a system. In this system, a digital video signal is applied to a discrete cosine transform (DCT) circuit 10 and to a video compressor 12. The transform coefficients developed by DCT 10 are applied to a perceptual model (PM) circuit 14 which develops an output signal reflecting the perceptual nature of the input signal. U.S. Pat. No. 5,113,256 discloses an exemplary embodiment of such a circuit. Video compressor 12 preferably comprises a motion predictive encoder of the type well known in the art (e.g. of the MPEG variety) which provides a compressed variable rate video output signal. The compressed video signal is applied to a buffer 16 which acts as a regulator to provide the compressed video signal at a fixed rate to a packetizer 18. In particular, a control signal representing the fullness of the buffer is fed back to compressor 12 and, together with an output of PM 14, controls the operation of the compressor to maintain the buffer at a nominal level of fullness necessary to support the fixed rate output signal provided thereby. That is, as buffer fullness decreases, compressor 12 is operated to effect a decreased level of compression so that more video data is generated to increase the fullness of the buffer. On the other hand, as buffer fullness increases, the level of compression is increased so that less video data is applied to the buffer. Finally, the packetized, compressed fixed rate video signal is applied to a transmitter 20 for transmission over a suitable channel.
Although not necessarily limited thereto, in the HDTV application of the foregoing system, it is anticipated that terrestial HDTV broadcasts will be effected over respective 6 MHz television channels and that, in addition to compressed video information, there is a great desire on the part of the broadcasters to further use the transmission system for auxiliary data services, such as for paging systems, the provision of stock quotes and the like. The income stream realized by leasing spectrum within the television channel for providing these auxiliary services may be used by the broadcasters to at least partially offset the expense of installing and operating the HDTV broadcast facility. In order to maximize this revenue stream, it is desirable to utilize as much of the channel spectrum as possible for auxiliary data services without at the same time adversely effecting the quality of the reproduced HDTV signal. Prior art systems of the type shown in FIG. 1 are not particularly well suited for this purpose because of the inflexibility occasioned by the fixed rate transmission of the video information.
It is therefore a basic object of the present invention to provide a television signal transmission system capable of maximizing the amount of auxiliary data which may be transmitted over a television channel without degrading the perceptual quality of the video image reproduced in response to the compressed video data received over the channel.
It is a more specific object of the invention to provide for the time multiplexed transmission of a compressed video signal and one or more auxiliary data signals, wherein the amount of compressed video information is adaptively controlled to insure satisfactory quality of the reproduced video image.